1. Field of the Invention
This invention relates to the assembly of rotary drill bits, and more particularly to a technique preferably employing a high energy beam to fuse drill bit segments together and to adjust the cutting elements of the drill bit to a precise and predetermined gage.
2. Introduction and Brief Description of Prior Art
Traditionally, rotary drill bits have been formed by joining a plurality of separately formed bit segments into an integral drill bit assembly. For many years the drill bit segments were joined together by welding processes using filler material. Recently, high energy electron and laser beams have been employed for fusing the bit segments together. Exemplary disclosures of high energy beam fusing techniques are found in U.S. Pat. Nos. 3,907,191; 3,987,859; 4,043,411; 4,045,646; 4,054,772; and 4,098,448. The disclosures contained within these patents are incorporated herein by this reference.
The high energy beam fusing technique possesses many significant advantages. One significant advantage is that the bit segments are more completely joined together by beam fusion than by conventional welding processes. One disadvantage of the beam fusing technique is that significant difficulty has been experienced in positioning the bit segments relative to one another to obtain a precise gage or cutting diameter of the cutting elements of the assembled bit. In the prior art welding technique, filler material could be added between the confronting faces of the bit segments during assembly to obtain the proper gage. However with the electron beam technique no filler material can be utilized since the abutting and confronting faces of the bit segments must be directly fused together by the high energy beam.
The recognized procedure to attempt to obtain the proper gage, as exemplified by the aforementioned patents, is to scissor the separate bit segments until a desired gage is obtained. Scissoring the segments causes the abutting and confronting faces to skew slightly with respect to the axis of the resulting bit. Since the high energy beam fusing technique occurs within a vacuum chamber and is completely mechanically controlled, the fixture for holding and positioning the assembled drill bit segments must have the capability of aligning the skewed, abutting confronting segment faces with the path of the high energy beam. Consequently the positioning fixture and its associated operating means must be capable of very complex directional movement and rotational orientation to correctly position the confronting segment faces to thoroughly fuse them together. The tedious nature and complexity of the mechanisms required for positioning the abutting confronting faces, which become the fused seams in the assembled drill bit, and the loss of efficiency of bit assembly during the complex positioning process, are significant problems in the prior art. The cost of maintaining the extreme precision of manufacturing processes to allow the segments to be joined with the confronting faces abutting and yet produce the proper gage diameter is prohibitive.
Although the scissoring technique has been adequately successful in positioning the cutting means of said drill bit segments at a predetermined gage, the scissoring technique changes such important constraints and dimensions as the journal angle, the offset angle, and the cutter element vertical location on the assembled drill bit. The journal angle, offset angle, and particularly the cutter vertical location are very important in obtaining acceptable performance of the drill bit over an extended lifetime of use.
Other prior art of interest relative to the present invention is a known manual technique of "pulling bits to gage". Pulling bits to gage has been practiced for many years as a salvage method for drill bits which, when assembled, did not obtain acceptable gage tolerances. Essentially, pulling bits to gage is a hand welding technique in which a zone of metal was melted into the back arcuate surface of the leg portion of the drill bit. When the melted zone of metal cooled, thermal contraction resulted and the leg portion was deflected outward. The cutting elements attached to the leg portion were thus also moved outward. Through skill and experience, the individual practicing this technique might obtain acceptable results in moving the cutting elements to within a desired range of gage tolerances. However, results were typically erratic due to the inability to determine the amount and configuration of the zone to be welded or melted and the inability to correlate the amount of shrinkage in the melted zone to the desired amount of gage adjustment. Also, the relatively imprecise control over the metallurgical effects created has sometimes resulted in serious imperfections and deficiencies in structural strength characteristics of the drill bit material. The penetration depth of certain prior art welding processes is severely limited, and unlike electron or laser beam techniques, the welding processes are effective only at or near the surface of the joined materials.
Other techniques, disadvantages and limitations in the prior art are known. Those skilled in the art may recognize still further limitations and disadvantages in view of the desirable aspects of the present invention, but comprehension of the desirable aspects of this invention should not diminish the significance of many previous troublesome limitations in the prior art.